U.S. patent number 4,683,467 [Application Number 06/556,882] was granted by the patent office on 1987-07-28 for image registration system.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Malcolm Macaulay, William F. Singleton.
United States Patent |
4,683,467 |
Macaulay , et al. |
July 28, 1987 |
**Please see images for:
( Certificate of Correction ) ** |
Image registration system
Abstract
An image registration system for registering multiple images on
a screen is disclosed. A registration pattern is projected on the
screen along with the image to be registered and in a fixed
position in that image. A sensor apparatus for each image, having
three photodetectors disposed in a right angle relationship is
mounted on the screen at a predetermined location for determining
the position of the registration pattern on the screen. The
position of the registration pattern on the screen is determined by
comparing the outputs of the three photodetectors. Difference
signals are produced by the comparisons and are used to reposition
the projected image to bring it into registration. The sensor
apparatus are disposed so that when the registration patterns are
in register with them, the images are in register with each
other.
Inventors: |
Macaulay; Malcolm (Fullerton,
CA), Singleton; William F. (Balboa Island, CA) |
Assignee: |
Hughes Aircraft Company (Los
Angeles, CA)
|
Family
ID: |
24223210 |
Appl.
No.: |
06/556,882 |
Filed: |
December 1, 1983 |
Current U.S.
Class: |
345/9; 345/207;
348/189; 348/745; 348/E3.04; 348/E9.027 |
Current CPC
Class: |
H04N
3/22 (20130101); H04N 9/3194 (20130101); H04N
9/3185 (20130101) |
Current International
Class: |
H04N
9/31 (20060101); H04N 3/22 (20060101); G02B
027/02 () |
Field of
Search: |
;353/30,94
;358/3,60,64,88,231,237,239,10 ;340/700,705,706,709,710 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Position Sensing Photodetectors", United Detector Technology, 3939
Landmark Street, Culver City, CA 90230, (advertisement). .
"Quad High Performance Op Amp", Signetics, Mar. 1982 catalog, p.
89, NE/SE5514..
|
Primary Examiner: Brigance; Gerald L.
Assistant Examiner: Cordell; Ruffin B.
Attorney, Agent or Firm: Runk; T. A. Karambelas; A. W.
Claims
What is claimed is:
1. An image registration system for registering a plurality of
projected imsges, comprising:
a plurality of projection means, each of the plurality for
projecting a respective image and for adjusting the spatial
position of the image in response to applied position signals, each
image having a registration pattern at a predetermined relative
location therein;
a plurality of sensor means, each of which is associated with a
respective one of the plurality of projection means, for sensing
the spatial position of the registration pattern projected by the
associated projection means and for applying position signals
indicative thereof to the associated projection means, so as to
obtain a predetermined spatial orientation between the images;
wherein the sensor means comprises photodetector means comprising
three photodetectors arranged in a right angle relationship, with
the photodetector at the vertex of the right angle being the
reference photodetector, and the other two photodetectors being
directional photodetectors, each photodetector providing detection
signals in response to detection of registration pattern
illumination; and
the sensor means further comprises processing means for processing
the detection signals and forming the position signals thereby.
2. The image registration system of claim 1 wherein the sensor
means further comprises processing means for respectively comparing
the detection signals provided by the directional photodetectors to
the detection signal provided by the reference photodetector and
providing two difference signals thereby.
3. The image registration system of claim 1 further comprising
three cells, each of which encompasses a respective one of the
photodetectors, each having an opening for receiving illumination,
and having diffusion means for diffusing the illumination prior to
its reaching the photodetector.
4. The image registration system of claim 1 wherein the
registration pattern is rectangular in shape having sides equal in
length to the respective center-to-center distance between the
reference photodetector and each directional photodetector.
5. The image registration system of claim 2 wherein the processing
means further comprises normalizing means for normalizing the two
difference signals into the position signals.
6. The image registration system of claim 5 wherein the projection
means includes a cathode ray tube having beam centering/deflection
means for positioning the cathode ray tube image in response to the
position signals.
7. The image registration system of claim 6 wherein the normalizing
means comprises a bipolar current source which normalizes the
difference signals into predetermined current ranges for applying
to the beam centering/deflection means.
8. The image registration system of claim 7 wherein the bipolar
current source comprises a constant current generating circuit
coupled to the beam centering/deflection means and a current
control circuit also coupled to the beam centering/deflection means
and coupled to the processing means, the current control circuit
controlling how much current is applied to or drawn from the beam
centering/deflection means in response to the difference
signal.
9. The image registration system of claim 8 wherein the current
control circuit comprises a circuit for drawing current in the
range of zero to twice the amount of current generated by the
constant current generating circuit, in response to the difference
signal.
10. An image registration system for registering a plurality of
images projected onto a common screen, comprising:
a plurality of projection means for projecting the plurality of
separate images onto the common screen, each image having a
registration pattern disposed in a fixed orientation within the
image, and each projection means having respective cathode ray
tubes with beam centering/deflection coils;
a plurality of photodetector sensors, each of which is associated
with a respective one of the plurality of projection means, each
photodetector sensor comprising three photodetectors arranged in a
right angle relationship with the photodetector at the vertex of
the right angle being the reference photodetector, the other two
photodetectors being directional photodetectors, each photodetector
providing detection signals in response to the detection of
registration pattern illumination;
the plurality of photodetector sensors collocated with the screen
and having predetermined spatial relationships with each other such
that when the registration patterns are in register with the
associated sensors, then the images are in register with each
other;
a plurality of processing means, each of which is associated with a
respective one of the plurality of sensors, for providing two
difference signals by comparing the detection signal from the
reference photodetector to the detection signal of one of the two
directional photodetectors, thereby providing one difference
signal, and comparing the detection signal of the reference
photodetector to the detection signal of the other of the two
directional photodetectors, thereby providing a second difference
signal; and
a plurality of repositioning means, each of which is associated
with a respective one of the plurality of processing means, for
controlling the current of the beam centering/deflection coils of
the cathode ray tube in response to the difference signals, thereby
positioning the image of the CRT.
11. An image registration system for registering a projected image
with a reference position, comprising;
projection means for projecting an image and for adjusting the
spatial position of the image in response to applied position
signals, the image having a registration pattern at a predetermined
relative location therein; and
sensor means for sensing the spatial position of the registration
pattern and for applying position signals indicative thereof to the
projection means, so as to obtain a predetermined spatial
orientation between the image and the reference position;
wherein the sensor means comprises photodetector means comprising
three photodetectors arranged in a right angle relationship, with
the photodetector at the vertex of the right angle being the
reference photodetector, and the other two photodetectors being
directional photodetectors, each photodetector providing detection
signals in response to detection of registration pattern
illumination; and
the sensor means further comprising processing means for processing
the detection signals and forming the position signals thereby.
12. The image registration system of claim 11 wherein the sensor
means further comprises processing means for respectively comparing
the detection signals provided by the directional photodetectors to
the detection signal provided by the reference photodetector and
providing two difference signals thereby.
13. The image registration system of claim 11 further comprising
three cells, each of which encompasses a respective one of the
photodetectors, each having an opening for receiving illumination,
and having diffusion means for diffusing the illumination prior to
its reaching the photodetector.
14. The image registration system of claim 11 wherein the
registration pattern is rectangular in shape having sides equal in
length to the respective center-to-center distance between the
reference photodetector and each directional photodetector.
15. The image registration system of claim 12 wherein the
processing means further comprises normalizing means for
normalizing the two difference signals into the position
signals.
16. The image registration system of claim 15 wherein the
projection means includes a cathode ray tube having beam
centering/deflection means for positioning the cathode ray tube
image in response to the position signals.
17. The image registration system of claim 16 wherein the
normalizing means comprises a bipolar current source which
normalizes the difference signals into predetermined current ranges
for applying to the beam centering/deflection means.
18. The image registration system of claim 17 wherein the bipolar
current source comprises a constant current generating circuit
coupled to the beam centering/deflection means and a current
control circuit also coupled to the beam centering/deflection means
and coupled to the processing means, the current control circuit
controlling how much current is applied to or drawn from the beam
centering/deflection means in response to the difference
signal.
19. The image registration system of claim 18 wherein the current
control circuit comprises a circuit for drawing current in the
range of zero to twice the amount of current generated by the
constant current generating circuit, in response to the difference
signal.
Description
BACKGROUND OF THE INVENTION
The invention relates generally to image projection and more
particularly, to image registration.
In some systems, an image projected on a screen must have a
particular register, i.e., with a reference point or with another
image or images projected on the same screen. As an example, where
the images of two cathode ray tubes (CRT), each CRT image having a
different color, are to be projected onto a single screen to form a
single image having the color combination of both CRT images, the
screen registration accuracy requirement for the two images can be
very severe in order to avoid undesirable color fringing. The
registration accuracy requirement may require registration within
one picture element. In some systems where high resolution is
required, a registration accuracy requirement of registration
within one-fourth of a picture element may exist. For a CRT picture
which is four inches in length in one dimension and has one
thousand picture elements on that axis, a registration requirement
of registration within 0.001 inch may exist.
Since image registration has been affected by component drift due
to heat, warm-up times, age, use, etc., prior techniques invoked
sophisticated designs of electrical, optical and mechanical
components for stability in view of these factors. Even with these
techniques, component drift was not compensated for and
registration needed to be readjusted frequently. A further
technique was the application of thermostatically controlled
heaters to bring all components to a selected temperature which was
above the ambient temperature. This technique still does not
provide the registration accuracy required and the heat has an
adverse effect on component reliability.
SUMMARY OF THE INVENTION
It is a purpose of the invention to overcome the above described
problems and other problems by providing an image registration
system which senses the position of the image to be registered on
the screen and generates registration control signals in response
to the screen sensed position. It is also a purpose to provide
registration sensing and adjustments continually, automatically and
rapidly.
It is also a purpose of the invention to provide an image
registration system which is simpler and more accurate than prior
techniques.
It is also a purpose of the invention to provide an image
registration system which is relatively simple in construction and
inexpensive to manufacture.
It is also a purpose of the invention to provide an image
registration system which maintains image registration over a wide
range of operating conditions. In particular, an image registration
system relatively insensitive to wide temperature variation and
which compensates for other factors tending to cause lack of image
registration.
The foregoing purposes and other purposes are attained by the
invention wherein there is provided an image registration system
which senses the image position on the screen and provides
positional control signals for repositioning the image relative to
a screen reference point.
In particular, a photodetector apparatus which generates position
signals in response to incident illumination is mounted on the
screen. The photodetector apparatus has an illumination reference
position established within it and generates signals corresponding
to the position of incident illumination in relation to the
reference position. In one embodiment, three photodetectors are
mounted in a right angle relationship to each other. The vertex
photodetector is the reference photodetector which establishes a
reference position to which the two other photodetectors are
compared. A registration pattern is projected on the screen and its
illumination is detected by the photodetectors. The registration
pattern has a fixed positional relationship to the associated
projected image, thus by sensing the position of the registration
pattern the image position is also known. The signal output of the
two photodetectors are compared individually to the signal output
of the reference photodetector to develop difference signals
representative of the position of the registration pattern in
relation to the position of the reference photodetector. The lack
of difference signals indicates registration, whereas the presence
of difference signals indicates an out of registration
condition.
The difference signals are normalized as required for input to the
image producing device to result in repositioning of the image. In
the embodiment where a CRT image is the projected screen image, the
difference signals are normalized to a current range. The currents
representative of the difference signals are fed back to the
centering/deflection coils of the CRT to reposition the image in
accordance with the difference signals.
Other purposes, features and advantages of the invention will be
more readily understood by consideration of the accompanying
drawings taken in conjunction with the following detailed
description, wherein like reference numerals designate like
elements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram of an image registration
system in accordance with the invention;
FIG. 2 is a diagrammatical view of an arrangement of three
photodetectors and an incident registration pattern;
FIG. 3 is a perspective sectional view of one photodetector cell of
the arrangement of FIG. 2;
FIG. 4 is a schematic diagram of a difference signal generating
circuit usable in the invention; and
FIG. 5 is a schematic diagram of a bipolar current source usable in
the invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the figures with more particularity, FIG. 1 shows a
functional block diagram of an image registration system in
accordance with the invention. In the embodiment shown in FIG. 1,
the images from two cathode ray tubes (CRT) 10 and 12 are projected
on a common screen 16 by means of a liquid crystal light valve
(LCLV) and lens apparatus shown as block 14. The LCLV and lens
apparatus 14 are known in the art and are not described herein with
further specificity. For a reference to an LCLV and lens apparatus,
refer to U.S. Pat. No. 4,127,322 to Jacobson et al. In the
embodiment shown in FIG. 1, the image of one CRT 10 has a green
color and the image of the other CRT 12 has a red color, the
combination of the two forming the desired image. It is desired to
project the differently colored images from the two CRTs 10 and 12
onto the common screen 16. In order to avoid color fringing and to
have a clear picture on the screen 16, and to gain the advantage of
color mixing of the red and green images, accurate registration of
the two images on the screen 16 is required.
Registration of the two images from the two CRTs 10 and 12 is
accomplished in FIG. 1, wherein two detector assemblies 18 and 20
are mounted on the screen 16, one detector assembly for each CRT
image. These detector assemblies are substantially identical in
function. In one embodiment of a detector assembly usable in the
invention, the detector assembly 18 has three photodetectors 22, 24
and 26 arranged in a right angle relationship to each other. This
arrangement is diagrammatically shown in FIG. 2. In this
embodiment, the photodetector located at the vertex of the right
angle is considered the reference photodetector 24, and the other
two photodetectors are located equidistantly away from the
reference photodetector, on the 90.degree. legs. In FIG. 2, the
photodetector 22 is located above the reference photodetector 24
and is termed the vertical photodetector 22. The photodetector 26
is located to the left of the reference photodetector 24 and is
termed the horizontal photodetector 26. To the reference
photodetector 24, the other two photodetectors 22 and 26 are
compared. A registration pattern is included in the CRT image in a
predetermined positional relationship to the rest of the image, and
is projected on the screen from the associated CRT, which in this
case is CRT 10. Because of the fixed positional relationship of the
registration pattern to the CRT image, by locating the registration
pattern, the location of the CRT image will be known.
As is functionally shown in FIG. 1, the image and/or registration
patterns of the CRTs 10 and 12 are projected onto the screen 16.
The detector assembly 18 dedicated to that CRT 10 will sense the
illumination of the registration pattern when it is incident
thereon. The photodetectors of the detector assembly produce
photocurrents proportional to the amount of illumination detected
by the active area of the photodetector.
FIG. 2 shows a detector assembly 18 usable in FIG. 1 with a
registration pattern 28 incident thereon. As shown in FIG. 2, the
registration pattern 28 is incident more upon the horizontal
photodetector 26 than upon either the reference or the vertical
photodetectors 24 or 22, but is incident equally upon the reference
and vertical photodetectors 24 and 22. In the following
description, it is given that all photodetectors are identical,
i.e., they will all produce the same amount of photocurrent in
response to the same amount of incident illumination. Each
photodetector in FIG. 2 will produce photocurrent due to this
incident illumination of the registration pattern 28; however, the
horizontal photodetector 26 will produce more than either of the
other photodetectors 22 and 24. By comparing the photocurrents
produced by the horizontal and vertical photodetectors 22 and 26
individually to the reference photodetector 24, two difference
signals will be produced. In the case of FIG. 2, the horizontal
photodetector 26 will produce a greater photocurrent than the
reference photodetector 24 thus indicating that the registration
pattern 28 is off register in the left horizontal direction. The
vertical photodetector 22 will produce an amount of photocurrent
equal to the photocurrent produced by the reference photodetector
24 thus indicating that the registration pattern 28 is in register
in the vertical direction. If all photocurrents were equal, this
would indicate that the registration pattern was equally incident
upon the photodetectors. Then the centroid of the registration
pattern would be in registration with the centroid of the
photodetector arrangement. Thus with the arranagement of three
photodetectors shown in FIG. 2, the position of a registration
pattern in any direction relative to the reference position can be
determined.
To generate a difference signal in response to an off center
registration pattern in the above embodiment, the photocurrent of
the horizontal photodetector 26 is compared to the photocurrent of
the reference photodetector 24, and the photocurrent of the
vertical photodetector 22 is compared to the photocurrent of the
reference photodetector 24. This is done in blocks 30 and 32 as
shown by the functional block diagram of FIG. 1. Difference signals
are produced by the comparisons which have a polarity indicative of
which direction the registration pattern is out of register, and an
amplitude indicative of how far out of register the registration
pattern is. These difference signals are then normalized for use in
repositioning the image generated by the image producing device,
which in FIG. 1, is a CRT. Thus in FIG. 1, the difference signals
may be normalized into a selected current range for applying to the
CRT centering/deflection coils.
In functionally combining FIG. 2 into FIG. 1, three signal lines
will be connected between detector assembly 18 and block 30, one
line for each photodetector 22, 24 and 26. In block 30, the above
described photocurrent comparisons will occur, difference signals
will be produced and a normalization of those difference signals
into repositioning currents will occur, one repositioning current
for each of the photocurrent comparisons. These currents are
applied to orthogonal centering/deflection coils of the CRT 10 and
a repositioning of the image/registration pattern combination
occurs.
In the case shown in FIG. 1 where two separate images from two
separate CRTs are to be registered on the common screen 16, a
separate detector assembly is dedicated to each CRT. These detector
assemblies are located on the screen in a predetermined
relationship to each other so that the CRT images are in register
with each other when the centroids of their registration patterns
are individually in register with the centroids of their dedicated
detector assemblies. Thus, adjustments may be made to each CRT by
its own image registration circuitry for repositioning its
image.
By using a registration pattern and a detector assembly with each
CRT, the images may be held in register on the common screen 16
automatically. Reregistration adjustments are performed
automatically and continually. Temperature variations which
typically cause drift, such as system warm-up, are compensated for
since the position of the registration pattern, and therefore the
image, is continually readjusted if it moves off the reference
position. By projecting the registration patterns continually with
the associated images, the images will be constantly in register
since the image registration system continually readjusts the
positions of the CRT images as required.
A photodetector structure such as that shown in FIG. 3 is usable in
the invention. The photodetector 34 is located in a cell 36 having
opaque sides and an opaque bottom with a translucent cover 38. The
translucent cover 38 is typically diffusing material which is
located between the photodetector and the illumination to be
detected. A diffuser film-polyester such as LDF6503TR by the 3M
Company is usable. The diffusing material spreads out the incident
illumination to achieve a broader and more continuous detection
range from the photodetectors.
The cell shown in FIG. 3 has a square shape however other shapes
are usable. The height of the cell walls is such that direct
illumination from the registration pattern strikes the
photodetector, but extraneous light, such as from overhead room
lighting, is rejected at least partially. It has been found that
making the inside surfaces of the cell nonreflective can improve
the signal to noise ratio by absorbing extraneous light which
enters the cell at an angle.
A square registration pattern such as that figuratively shown in
FIG. 2 is usable in the invention. It has been found that a
registration pattern of ten horizontal picture elements by ten
vertical lines is usable. This results in a registration pattern of
ten picture elements by ten picture elements. Other patterns are
usable and generation of registration or test patterns by a CRT is
well known in the art and is not further described herein. The
relative sizes of the registration pattern and active detection
area of the detector assembly should be such that movement of the
registration pattern can be detected. A registration pattern which
is somewhat smaller than the active area of the detector assembly
is necessary to avoid a situation where the registration pattern
has moved out of register but this movement is not detectable by
the photodetectors. A registration pattern having a side length
which is approximately equal to the center-to-center length between
the reference photodetector and one of the other photodetectors has
been found to avoid the above situation. This is shown in FIG. 2
where the registration pattern 28 is square and the side length is
equal to the distance between the centers of the vertical
photodetector 22 and the reference photodetector 24.
Where the size of the registration pattern is fixed, such as fixed
at ten horizontal picture elements by ten vertical lines, the size
of the active detection area of the detector assembly may require
adjustment depending upon the screen size of the registration
pattern. Where the image and associated registration pattern have
been projected onto a large screen and enlarged to fill that
screen, the center-to-center distances between the reference
photodetector and the other two photodetectors may require
lengthening to retain the relative size relationship between the
registration pattern and the active detection area of the detector
assembly. Conversely where the registration pattern is smaller,
such as when projected onto a small screen, the active detection
area of the detector assembly may need to be contracted.
A difference signal generating circuit usable in the invention is
shown in FIG. 4. As is shown, the photocurrents from the three
photodetectors 22, 24 and 26 are compared in two operational
amplifiers 46 and 48 to produce difference signals 50 and 52. In
particular, a signal from the reference photodetector 24 is
supplied to the same polarity inputs of both operational amplifiers
46 and 48. The opposite polarity input of one operational amplifier
46 is supplied with the signal from the horizontal photodetector 26
while the opposite polarity input of the other operational
amplifier 48 is supplied with the signal from the vertical
photodetector 22. Feedback capacitors 54 (1 .mu.f) and feedback
resistors 56 (10M ohms) are used for integration and gain purposes.
Thus, amplified and integrated difference signals 50 and 52 are
supplied at the outputs of the operational amplifiers 46 and
48.
Although previously described as being indentical, in practicality,
photodetectors rarely are. Thus, a nulling technique is included as
a part of the circuit shown in FIG. 4. When a registration pattern
is in register on the detector assembly, the circuit shown in FIG.
4 is adjusted so that the difference signals 50 and 52 are zero.
Variable resistors 58 (0-25K ohms) are supplied for this purpose.
Thus, when the image is in register, the variable resistors 58 are
adjusted until the difference signals 50 and 52 are zero. This
compensation technique nulls out the differences between the
photodetectors as well as between the other circuit components.
This nulling technique also compensates for extraneous light
striking the photodetectors.
Resistors 98 are 91K ohms, resistors 100 are 100K ohms and
resistors 102 are 0.1M ohms. Power source 104 is +15 V.sub.dc and
power source 106 is -15 V.sub.dc.
Photodetectors usable in the circuit of FIG. 4 include the MRD 711
by Motorola Inc., Phoenix, Ariz. Operational amplifiers usable
include the LM 741 by National Semiconductor Corp., Santa Clara,
Calif. A different photodetector device may be used in place of the
photodetectors 23, 24 and 26 shown in FIG. 4. A position sensing
photodetector, such as the "SC" series by United Detector
Technology, Culver City, Calif., provides X and Y axes positions
from a reference point established within it.
In the embodiment where the difference signals themselves are not
directly usable, such as in the CRT embodiment shown in FIG. 1, a
normalization of those signals is effected. For a normalization of
the difference signals 50 and 52 into a bipolar current range, the
circuit 63 shown in FIG. 5 has been found to be usable. The bipolar
current source 63 shown provides a current of .+-.twenty
milliamperes from an input of .+-.ten volts to a
centering/deflection coil 60 of the CRT to reposition the image of
that CRT.
In the circuit 63 shown in FIG. 5, an output, such as a difference
signal 50 from the circuit shown in FIG. 4, is coupled to the input
resistor 62 (3K ohms). The upper circuit part 64 of the bipolar
current source 63 is a constant current source of .+-.twenty
milliamperes which is output to a node 68. The lower circuit part
70 of the bipolar current source 63 draws current from the same
node 68 in the range of zero to forty milliamperes as controlled by
the difference signal 50. Therefore, current in the range of
.+-.twenty milliamperes is present at the node 68 and on the line
72 to or from the centering/deflection coil 60. Zener diode 74 is a
1N757A and zener diode 76 is a 1N3826A, both made by National
Semiconductor. Transistor 78 is a 2N2905A, transistor 80 is a
2N2219A and diodes 82 are IN4150, all available from Motorola Inc.
Resistors 84 are 100 ohms, resistor 86 is 470 ohms, resistor 88 is
3K ohms, resistor 90 is 120 ohms and resistor 92 is 1K ohms. Power
source 94 is .+-.30 V .sub.dc while power source 96 is -15
V.sub.dc.
When used with the difference signal generating circuit of FIG. 4,
two bipolar current sources as shown in FIG. 5 are required, each
coupled to a different difference signal and to an orthogonal
centering/deflection coil.
Thus there has been shown and described, a new and useful image
registration system. Although the invention has been described in
detail, it is anticipated that modifications and variations may
occur to those skilled in the art which do not depart from the
inventive concepts. It is intended that the invention be limited
only by the scope of the claims, not by the description, and so the
invention will include such modifications and variations unless the
claims limit the invention otherwise.
* * * * *